Analysis of biological activity of a toxin protein, TcaB isolated from Photorhabdus-Heterorhabditis symbiont, against agriculturally important insects.
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Date
2020
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DIVISION OF NEMATOLOGY ICAR-INDIAN AGRICULTURAL RESEARCH INSTITUTE NEW DELHI
Abstract
Insect-parasitic nematodes have long been used as biological control agents against
different agriculturally important pests, and the insecticidal bacteria they harbor have been
investigated in detail. Photorhabdus akhurstii produce a multitude of insecticidal proteins
that help this bacterium and its symbiotic partner, Heterorhabditis indica to kill the insects.
Herein, a previously characterized TcaB toxin from P. akhurstii strains IARI-SGHR2 and
IARI-SGMS1 were assessed for their biological activity against the agriculturally important
pests, Helicoverpa armigera, Spodoptera litura and S. frugiperda, followed by analyzing
their immunomodulatory activity, cytotoxicity and mode of action in test insects. When
injected in hemocoel, the injection LD50 values of TcaB of either strain (at 48 h post
incubation) was comparatively higher in H. armigera (190.83-200.65 ng/g), S. litura (181.8-
195.98 ng/g) and S. frugiperda (92.45-105.6 ng/g) than the reference insect, Galleria
mellonella (34.32-42.87 ng/g). Similarly, force feeding of the toxin of either strain (at 48 h
post incubation) exhibited higher oral LD50 values for H. armigera (186.2-196.95 ng/g), S.
litura (173.45-188.05 ng/g) and S. frugiperda (80.55-95.63 ng/g) than G. mellonella (30.42-
39.27 ng/g). This was attributed to the greater number of hemocyte cells in control larvae
(PBS buffer injected) of H. armigera (76.8-77.5 × 105
/ml), S. litura (106.7-109.8 × 105
/ml)
and S. exigua (54.9-55.5 × 105
/ml) than G. mellonella (2.46-2.48 × 105
/ml). However,
injection of recombinant TcaB caused significant reduction in hemocyte numbers and
viability in all the test insects at 12 h post incubation. In addition, induction of phenol
oxidase-mediated immune response was documented in all the insects at 6 h.
Histopathological studies indicated that TcaB toxin had caused extensive destruction of
midgut epithelial cells alike of Bt toxins. The findings from the present study provide an
impetus in search for alternative of Bt-based pesticides which is rapidly losing its relevance
in global agriculture due to development of resistance in insect pests.
Keywords: EPN, biological control, Photorhabdus akhurstii, insecticidal protiens,
TcaB, cytotoxicity, immunomodulatory activity, histopathological studies.
Description
T-10456